WIP: Add combo box for choosing the example plots

At least for the plot examples it would be helpful to have a way to select a specific plot or to switch between them, for example via a combobox. See the following commit to get an idea what I mean by that. I didn't pay attention to any memory leaks introduced by switching to another plot (and there should be many, unless you magically free all those QCP Items that are instantiated inside all the different setup methods (for the examples).